Showery Sunday & a Look at the Week Ahead

Anyone in the Sacramento area catch the thunderstorms Thursday night? Quite the show from here in Rocklin! Below is a radar image from when storms just began developing south of Yuba City, between Yuba City and Wheatland. Shortly after the below radar scan, the line expanded across the valley and up into the foothills, spreading south along a band of convergence and weak instability promoted by a weak upper low sliding inland.

Radar image from around 8:30pm Thursday evening.

20 second exposure in which I caught a cloud-to-cloud lightning strike around 9:30pm, looking north from Rocklin towards Lincoln.

We have one last system to squeeze out of this trough before it exits, which is actually just offshore today (Saturday). A couple days ago it appeared this surface low within the cold trough in the eastern Pacific would cut off and drop south just along the California coast, keeping most of the system’s associated precipitation just offshore and along the coast with it. However, now, it appears the low will be close enough to the coast to generate some lift under the diffluence the low has to offer pretty far inland, though, a lack of deep moisture will limit the amount of precipitation that can develop inland compared to areas along the coast, with the brunt of the precipitation falling offshore, unfortunately for us.

Offshore, the low sits about 400 miles off the Oregon coast, and is currently tracking south-southwest at about 15 – 20 mph based on a rough satellite track. This puts the center about 100 – 200 miles off the SFO coast Sunday morning. That 100 – 200 mile variation is based off a bunch of model variations. Latest 18z NAM indicates a distance between the low and land of about 140 miles, which puts the precipitation band right over the coast, giving areas west of Davis, Tracy, and generally south of Eureka. Precipitation amounts of around .10 – .25″ seem possible along the coast, locally upwards of a half inch, but QPFs of 1 – 1.25″ remain just offshore. Inland, showers seem most possible in the mountains/sierra, where lift is maximized due to upsloping. A few showers can’t ruled out in other parts of northern California as well, but generally most of the precipitation should be confined to the mountains and lower elevations west of the delta.

A cool northerly flow takes over Monday and continues through most of the workweek as the Pacific jet essentially sits directly over northern California, transporting cold air from the Gulf of Alaska and Canada into the western U.S., with cold upper low developing in inland WA/OR into ID and western MT through the day Wednesday, remaining over the region through Friday before ejecting east ahead of a deepening low in the east-central Pacific.

The first wave of colder air involves 850mb temperatures of -1 to -2c Monday into Tuesday, before a reinforcing wave of cold air surges south as the upper low develops in the northern Rockies, directing cold air to its west south into California. This second wave includes -3 to -6c 850mb temps, which surge overhead Thursday into Friday. Mid-level temperatures such as these will support highs in the valley in the upper 40s to upper-50s through the workweek, with overnight lows in the 20s and 30s… of course a good 10 – 30 degrees colder in the sierra.

Models are all over the place precipitation wise. For instance, the GFS has a weak surface low develop in the strong northwesterly jet wrapping around the upper low in the northern Rockies, which brings some scattered light precipitation Wednesday night into Thursday as the low tracks inland. The ECMWF doesn’t indicate any westerly waves to enhance precipitation chances, but does indicate some mountain/sierra showers periodically from Wednesday to Friday as cold air aloft and weak lift supports higher elevation convective development. If the GFS is right, some snow would be possible down to around 3500 – 4000ft, higher than that of the ECMWF’s levels at the same time due to the fact the GFS’s surface low picks up a bit of moisture which modifies the airmass a bit.

I’ll write up another update between Monday and Wednesday regarding the cold mid-week system(s).

Beyond Thursday there is increasing potential for a stronger, wetter storm between Saturday and early next week as a deep low between Alaska and Hawaii in the central Pacific deepens and taps into a deep sub-tropical moisture plume from the Hawaii region, pulling the low northeastward towards some colder air, creating a favorable environment for cyclongenesis along the frontal/moisture band. When a new low forms, the trough and associated deep low begins tracking eastward toward the western U.S. coast, sending a strong cold front, supported by a moderate strength upper level jet and precipitable water plume in excess of 1”. Models vary significantly with the strength, track, and timing of the system, with the strength and track being the main variables that go into how this system affects us in northern California. The GEM is the most bullish model, indicating a large GOA low with several “child” lows rotating around it, each one running into the deep moisture plume, maximizing precipitation potential in the western U.S. The GFS and ECMWF are relatively close to one-another, but do vary with the strength.

I’ll have additional updates this week regarding the potentially wet pattern later in the week, and if needed on the colder systems early this week. Stay tuned!

[…] came out of nowhere and turned out to be the biggest rain we’ve had this season. There had been some chatter on the weather blogs that we might get a few showers, but I didn’t see anyone predicting that it […]

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